1. Field of the Invention:
The present invention relates to a soft magnetic steel material and, more specifically, it relates to a soft magnetic stainless steel having good cold forgeability together with good magnetic property, electric property, corrosion resistance and machinability which is suitable for material for use in electronic fuel injection devices, solenoid valves, magnetic sensors, etc.
2. Description of the Prior Art:
Heretofore, 0.1%C steels have been used in most of magnetic core materials for use in electronic fuel injection devices, solenoid valves, magnetic sensors, etc., because 0.1%C steels have soft magnetic property to some extent, as well as good cold forgeability that can be cold forged easily even into complicate shapes such as those of parts for the above-mentioned application uses, and production cost and material cost are inexpensive.
On the other hand, there has been a demand in recent years for those steel materials concurrently having the following three characteristics: capability of production in existent fabrication line used for 0.1%C steels, that is, having forgeability as comparable with that of 0. 1%C steels, excellent corrosion resistance and, further, improved magnetic response (compliance of the material to external magnetic fields) in view of the demand for further higher performance. However, these demands can be satisfied only to the following extent by the existent technics at present.
At first, 0.1 % C steels are provided with corrosion resistance by applying Ni-P plating after cold forging. Although the materials have excellent cold forgeability (tensile strength 32 kgf/mm.sup.2), they involve a drawback, when incorporated as a part in a device and used, in that platings are defoliated to cause clogging in valves during operation of the device to they are assembled. In addition, the materials have a drawback that the electrical resistance is as low as 15 .mu..phi.cm and the magnetic response is extremely poor. Further, referring to stainless steels, Fe-13Cr-1Si-0.25Al steels developed in the latter half of 1970's have been used since ten years or so as the materials of excellent cold forgeability. Although the materials have excellent corrosion resistance, high electric resistance and excellent response, the tensile strength is as high as 45 Kgf/mm.sup.2 and, accordingly, they can not be compared with 0.1%C steels at all (tensile strength of 32 kgf/mm.sup.2, and critical compressibility of 70%). Accordingly, the materials can not be cold forged in the fabrication steps used for 0.1%C steels. Subsequently, although improvement have been tried for the cold forgeability and the electromagnetic properties of 13Cr-1Si-0.25Al steels, those materials having cold forgeability superior to 13Cr-1Si-0.25Al have not yet been developed. In addition, since the material has fatigue strength at welded portion of as low as 25 kgf/cm.sup.2, it can not satisfy the required quality of that of greater than 100 kgf/cm.sup.2.